Ceramic Honeycomb Structure Skin Coating

ABSTRACT

A porous ceramic (honeycomb) structure skin coating and a method of producing a porous ceramic structure skin coating which provides a hardshell, strong, acid- and alkali-resistant, chip-resistant ceramic honeycomb structure coating which resists pollution control catalyst from being absorbed into the skin coating.

This application claims the benefit of the filing date, under 35 U.S.C.§119(e), of U.S. Provisional Application for Patent Ser. No. 61/122,583,filed on Dec. 15, 2008, which is incorporated herein by reference as iffully written out below.

BACKGROUND

Ceramic honeycomb structures, such as those used as catalytic convertersand diesel particulate filters (“DPFs”), are manufactured by variousprocesses. Generally, the honeycomb structures are manufactured byextrusion, resulting in a multiplicity of through holes or passageswhich are separated by the walls of the honeycomb structure. Eachpassage is sealed at either the inlet or outlet end of the structure andthe structure is fired at a high temperature. Adjacent passages arecapped alternatively, forming a checkerboard pattern, so that a fluidpassing into the structure will be forced to pass through a wall of thestructure before passing out of the structure. In this manner, the fluidpassing through the structure can either be contacted by a catalyst orparticles in the fluid can be filtered, as the fluid passes through thewalls of the honeycomb structure.

The catalysts which are used with those honeycomb structures incatalytic converters require high temperatures and high porosity of thehoneycomb walls in order to ensure an efficient rate of catalysis. It istherefore necessary that the structure be able to heat up quickly inorder to effectively clean exhaust from an engine which has just beenstarted. Those structures which are used as DPFs require that there below pressure loss as the exhaust passes through the filter, since DPFsare usually utilized in circumstances where the exhaust will passthrough the DPF, and then through an independent catalytic converter.

Therefore, it is desired that such honeycomb structures, while beingable to withstand the extreme temperatures associated with combustionengines, have a low heat capacity and that the pressure loss through thestructure is minimized. In order to achieve these properties, a highporosity and low wall thickness are desirable. However, high porosityand low wall thickness result in low mechanical strength, which resultsin various problems during production.

In an attempt to rectify these problems, it is now the state of the artto enclose the honeycomb structure within a ceramic paste or mat whichwill lend the structure increased mechanical strength, protection fromvibration, and seal the structure so that, when it is canned, exhaustgases will not pass between the structure and its housing.

It has also been proposed to manufacture multiple smaller honeycombstructures and bond them together using a ceramic adhesive material tocreate a single structure, which will still require the use of a skincoating around the exterior of the structure to ensure uniformity of theexterior surface. These single honeycomb structures are able to supporttheir own weight more effectively, and the adhesive material lends thestructure increased mechanical strength once the monolith is fired.

Whether the monolith is assembled from smaller honeycomb structures oris extruded as a single unit, the exterior of the structure may requiremachining after the firing step to meet the tight specificationtolerances for roundness and actual diameter in the shape of thestructure, and to create a surface which will adhere to the skincoating. In some instances, this machining will result in partialhoneycomb cells being exposed, which will need to be filled by the skincoating, usually a ceramic paste in these instances.

Skin coatings comprising ceramic pastes are desirable because they canbe made of materials similar to that of the ceramic honeycomb structure,resulting in similar heat capacities, and they can be used to perfectthe shape of the structure. Mats can be used in conjunction with pastesto provide addition protection from vibration damage to the structurewhile it is in use. Desirably, the pastes will resist cracking, peeling,and degradation by absorption of acidic catalytic substances. None ofthe previously proposed ceramic paste materials have sufficientlyachieved all of these goals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of the green density of examples ofthe subject skin coating formulation compared to a commercial ceramicpaste skin coating product.

FIG. 2 is a graphical representation of the viscosity of examples of thesubject skin coating formulation compared to a commercial ceramic pasteskin coating product.

FIG. 3 is a graphical representation of the modulus of rupture undervarious conditions of examples of the subject skin coating compared to acommercial ceramic paste skin coating product.

DETAILED DESCRIPTION

We have now shown that addition of a secondary fiber can minimizecracking of the skin coating during the drying and firing stages ofmanufacture of a ceramic honeycomb structure. These secondary fibersneed not necessarily be high temperature resistant fibers; fibers thatare not resistant to particularly high temperatures function very wellin preventing cracking during the drying and firing stages of the skincoating. The term “honeycomb structure” includes any porous ceramicstructure utilized in exhaust gas treatment devices, such as catalyticconverters, diesel particulate filters, selective catalyst reductionunits, NO_(x) traps, and the like.

Further, we have shown that, during the drying stage, a non-absorbent,hard, dense, eggshell-like surface forms on the surface of the skincoating as herein described. While not being limited by theory, it isbelieved that this surface is formed by the migration of the silicaspecies during the drying stage. This surface prevents the acidiccatalyst coating from being absorbed into the skin coating. Preventingabsorption is desirable because, as described above, the skin coatingmay be degraded by exposure to the acidic catalyst coating. Preventingabsorption also allows for the use of a lesser quantity of catalystcoating, reducing overall production costs.

Provided is a ceramic honeycomb structure skin coating and a method ofproducing a ceramic honeycomb structure skin coating which provides ahardshell, acid- and alkali-resistant, chip-resistant ceramic honeycombstructure skin coating having high strength, and which resists pollutioncontrol catalysts being absorbed into the skin coating.

In one embodiment, the ceramic skin coating material for porous ceramic(e.g., honeycomb) substrates comprises refractory ceramic fiber orbiosoluble inorganic fiber; a viscosity modifier; a colloidal inorganicoxide; optionally, an inorganic binder; optionally, an inorganicparticulate; and, optionally, a secondary inorganic fiber.

The refractory ceramic fibers or biosoluble inorganic fibers maycomprise at least one of aluminosilicate fibers, alkaline earth silicatefibers, or calcium aluminate fibers. The refractory ceramic fiber (RCF)may include but not be limited to aluminosilicate fibers. The alkalineearth silicate fibers may include but not be limited to magnesiumsilicate fibers or calcium magnesium silicate fibers.

These primary fibers (RCF or biosoluble inorganic fibers) may beutilized with various degrees of shot content, ranging from “as is” (asproduced) to high index and air classified fibers, in whichsubstantially all shot has been removed. In certain embodiments, theprimary fiber may be ball milled.

The viscosity modifier may include but not be limited to alkyl cellulosepolymers, such as methyl cellulose (MC) and/or its derivatives, such ashydroxypropyl methyl cellulose (HPMC), hydroxyethyl methyl cellulose(HEMC), hydroxyethylcellulose (HEC), carboxymethylcellulose (CMC),hydroxyethylcarboxymethylcellulose (HECMC), orcarboxymethylhydroxyethylcellulose (CMHEC), or mixtures thereof. Incertain embodiments the viscosity of the viscosity modifier is withinthe range of about 20 cps to about 2000 cps.

Other non-limiting examples of viscosity modifiers include polyalkyleneoxides, certain polysaccharides, polyacrylic acids, polyacrylamides, andmixtures thereof. The polyalkylene oxide may include, but not be limitedto, polyethylene oxides having molecular weights ranging from about 1million to about 4 million g/mol. Illustrative examples of suitablepolysaccharides include welan gum, diutan gum, xanthan gum and mixturesthereof. The polyacrylic acid may have a molecular weight of about500,000 g/mol or greater.

The colloidal inorganic oxide may be colloidal silica, colloidalalumina, colloidal zirconia or mixtures thereof. Colloidal silica, suchas those available from Nalco Chemical Company, are stable dispersionsof nanometer size silica particles in water or other liquid medium.Colloidal silica particle sizes may range from about four to about 100nanometers in diameter. The colloidal silica may be stabilized, such aswith sodium or ammonium ions, and may have a pH range of about 2 toabout 12.

The inorganic particulate may include, but not be limited to, at leastone of alumina, cordierite (such as cordierite grog), mullite, titania,aluminum titanate, or silicon carbide. The inorganic particulate may beselected to include at least one component which has a thermal expansioncoefficient which is compatible with the thermal expansion coefficientof the ceramic honeycomb substrate to which the skin coating is to beapplied. Inorganic particulate particle sizes may be about 300 micron orless, in certain embodiments less than about 100 microns.

The inorganic binder may comprise clay. The clay may be calcined oruncalcined, and may include but not be limited to attapulgite, ballclay, bentonite, hectorite, kaolininte, kyanite, montmorillonite,palygorskite, saponite, sepiolite, sillimanite, or combinations thereof.Inorganic binder particle sizes may be about 150 microns or less, incertain embodiments less than about 45 microns.

The secondary inorganic fibers may include but not be limited to glassfibers, leached silica fibers, high alumina fibers, mullite fibers,magnesium aluminosilicate fibers, S-2 glass fibers, E-glass fibers, orfine (sub-micron) diameter alumina-silicate fibers (HSA) and mixturesthereof.

In addition to the secondary inorganic fibers, organic binder fibers mayoptionally be included in the skin coating formulation. Suitableexamples of binder fibers include polyvinyl alcohol fibers, polyolefinfibers such as polyethylene and polypropylene, acrylic fibers, polyesterfibers, ethyl vinyl acetate fibers, nylon fibers and combinationsthereof. These fibers may be used in amounts ranging from 0 to about 10percent by weight, based upon 100 percent by weight of the totalcomposition.

Other organic binders or resins may be optionally included in the skincoating formulation. Examples of suitable organic binders or resinsinclude, but are not limited to, aqueous based latexes of acrylics,styrene-butadiene, vinylpyridine, acrylonitrile, vinyl chloride,polyurethane and the like. Silicone latexes are also suitable. Otherresins include low temperature, flexible thermosetting resins such asunsaturated polyesters, epoxy resins and polyvinyl esters (such aspolyvinylacetate or polyvinylbutyrate latexes). Up to about 10 percentby weight organic binder or resins may be employed. Solvents for thebinders, if needed, can include water or a suitable organic solvent,such as acetone, for the binder utilized. Solution strength of thebinder in the solvent (if used) can be determined by conventionalmethods based on the binder loading desired and the workability of thebinder system (viscosity, solids content, etc.).

Refractory ceramic fiber typically substantially comprises alumina andsilica, and typically contain from about 45 to about 60 percent byweight alumina and from about 40 to about 55 percent by weight silica.RCF fiber length is typically less than about 5 mm, and their averagefiber diameter may range from about 0.5 μm to about 10.5 μm. FIBERFRAX®refractory aluminosilicate ceramic fibers (RCF), are available fromUnifrax I LLC, Niagara Falls, N.Y.

The term “biosoluble inorganic fiber” refers to fibers that aresubstantially decomposable in a physiological medium or in a simulatedphysiological medium such as simulated lung fluid, saline solutions,buffered saline solutions, or the like. The solubility of the fibers maybe evaluated by measuring the solubility of the fibers in a simulatedphysiological medium as a function of time. Biosolubility can also beestimated by observing the effects of direct implantation of the fibersin test animals or by the examination of animals or humans that havebeen exposed to fibers, i.e. biopersistence. A method for measuring thebiosolubility of the fibers in physiological media is disclosed in U.S.Pat. No. 5,874,375 assigned to Unifrax I LLC.

Another approach to estimating the biosolubility of fibers is based onthe composition of the fibers. For example, Germany classifiesrespirable inorganic oxide fibers based on a compositional index (KIvalue). The KI value is calculated by a summation of the weightpercentages of alkaline and alkaline-earth oxides and subtraction of twotimes the weight percent of aluminum oxide in inorganic oxide fibers.Inorganic fibers that are biosoluble typically have a KI value of about40 or greater.

Without limitation, suitable examples of biosoluble inorganic fiber thatcan be used to prepare the present skin coating material include thosebiosoluble inorganic fibers disclosed in U.S. Pat. Nos. 6,953,757;6,030,910; 6,025,288; 5,874,375; 5,585,312; 5,332,699; 5,714,421;7,259,118; 7,153,796; 6,861,381; 5,955,389; 5,928,975; 5,821,183; and5,811,360, each of which are incorporated herein by reference.

The biosoluble alkaline earth silicate fiber may comprise thefiberization product of a mixture of oxides of magnesium and silica,commonly referred to as magnesium-silicate fibers. Themagnesium-silicate fibers generally comprise the fiberization product ofabout 60 to about 90 weight percent silica, from greater than 0 to about35 weight percent magnesia and 5 weight percent or less impurities.According to certain embodiments, the alkaline earth silicate fiberscomprise the fiberization product of about 65 to about 86 weight percentsilica, about 14 to about 35 weight percent magnesia, 0 to about 7weight percent zirconia and 5 weight percent or less impurities.According to other embodiments, the alkaline earth silicate fiberscomprise the fiberization product of about 70 to about 86 weight percentsilica, about 14 to about 30 weight percent magnesia, and 5 weightpercent or less impurities.

Illustrative examples of the biosoluble inorganic fiber include, but arenot limited to, ISOFRAX® alkaline earth silicate fibers, having anaverage diameter of between about 0.6 microns and about 2.6 microns,available from Unifrax I LLC, Niagara Falls, N.Y. Commercially availableISOFRAX® fibers generally comprise the fiberization product of about 70to about 80 weight percent silica, about 18 to about 27 weight percentmagnesia and 4 weight percent or less impurities.

Alternatively or additionally, the biosoluble alkaline earth silicatefiber may comprise the fiberization product of a mixture of oxides ofcalcium, magnesium and silica. These fibers are commonly referred to ascalcia-magnesia-silicate fibers. The calcia-magnesia-silicate fibersgenerally comprise the fiberization product of about 45 to about 90weight percent silica, from greater than 0 to about 45 weight percentcalcia, from greater than 0 to about 35 weight percent magnesia, and 10weight percent or less impurities.

Suitable calcia-magnesia-silicate fibers are commercially available fromUnifrax I LLC (Niagara Falls, N.Y.) under the registered trademarkINSULFRAX. INSULFRAX® fibers generally comprise the fiberization productof about 61 to about 67 weight percent silica, from about 27 to about 33weight percent calcia, and from about 2 to about 7 weight percentmagnesia. Other commercially available calcia-magnesia-silicate fiberscomprise about 60 to about 70 weight percent silica, from about 25 toabout 35 weight percent calcia, from about 4 to about 7 weight percentmagnesia, and optionally trace amounts of alumina; or, about 60 to about70 weight percent silica, from about 16 to about 22 weight percentcalcia, from about 12 to about 19 weight percent magnesia, andoptionally trace amounts of alumina.

Biosoluble calcium aluminate fibers are disclosed in U.S. Pat. No.5,346,868, U.S. Patent Publication No. 2007-0020454 A1, andInternational Patent Publication No. WO/2007/005836, which areincorporated herein by reference.

With respect to the secondary fibers, other alumina/silica ceramicfibers, such as high alumina or mullite ceramic fibers, may be made bysol gel processing, and usually contain more than 50 percent alumina. Anexample is FIBERMAX® fibers, available from Unifrax I LLC of NiagaraFalls, N.Y. Magnesia/alumina/silicate fiber such as S2-GLASS, arecommercially available from Owens Corning, Toledo, Ohio. S2-GLASS fiberstypically contain from about 64 to about 66 percent silica, from about24 to about 25 percent alumina, and from about 9 to about 10 percentmagnesia.

Leached silica fibers may be leached in any manner and using anytechniques known in the art. Generally, leaching may be accomplished bysubjecting glass fibers to an acid solution or other solution suitablefor extracting the non-siliceous oxides and other components from thefibers. A detailed description and process for making leached glassfibers high in silica content is contained in U.S. Pat. No. 2,624,658,the entire disclosure of which is incorporated herein by reference.Another process for making leached glass fibers high in silica contentis disclosed in European Patent Application Publication No. 0973697.

Leached glass fibers are available under the trademark BELCOTEX fromBelChem Fiber Materials GmbH, Germany, under the registered trademarkREFRASIL from Hitco Carbon Composites, Inc. of Gardena California, andunder the designation PS-23 (R) from Polotsk-Steklovolokno, Republic ofBelarus.

In another embodiment, a method of producing a porous ceramic(honeycomb) structure skin coating is provided comprising forming amixture of: ceramic fibers or biosoluble inorganic fibers; a viscositymodifier; a colloidal inorganic oxide; optionally, an inorganic binder;optionally an inorganic particulate; and optionally secondary inorganicfibers.

In one embodiment, the dry ingredients are combined in one part, andseparately the wet ingredients (colloidal inorganic oxide and water) arecombined in a second part, and then both parts are mixed together. Inanother embodiment, the dry ingredients may be added to the wetingredients in any order, and mixed. The skin coating material may bedried, for example, at about 50° to about 10° C. for about two hour, oruntil completely dry. The dried skin coating material may be fired atabout 500-1100° C. for about 1 to about 5 hours, optionally with aheating and cooling rate of about 100° C./hr or less.

In the production of an exhaust gas treatment device, after theskin-coated ceramic honeycomb structure is fired, the honeycomb may besoaked in a catalyst containing acidic or basic solution or dispersion,and subsequently dried and re-fired.

In certain embodiments, a skin coating material for porous ceramic(honeycomb) substrates is provided, comprising: refractory ceramic fiberor biosoluble inorganic fiber; a viscosity modifier; a colloidalinorganic oxide; an inorganic binder; an inorganic particulate; and, asecondary inorganic fiber.

EXAMPLES

Examples of various subject skin coating formulations (Examples A, B andC) are set forth in Table 1 below. These were tested in comparison to acommercial ceramic paste product that is used as a DPF skin coatingformulation.

TABLE 1 Ingredient Ex. A % Ex. B % Ex. C % Fiber-RCF QF grade 0  0.00%200  38.76% 100  20.41% (Ball milled) Fiber-RCF Air 100  21.21%  0.00% 0.00% Classified Cordierite 140  29.69% 140  25.74% 140  28.57%Calcined Kaolin 20  4.24% 10  1.84% 20  4.08% E-glass-⅛″ 0  0.00%  0.00%3.5  0.71% E-glass- 1/16″  0.00% 2.5  0.46%  0.00% Methyl Cellulose 1.5 0.32% 1.5  0.28% 1.5  0.31% Colloidal silica 90  7.64% 80  5.88% 125 10.20% Water 120  36.90% 110  29.04% 100  35.71% Mass Solids 472100.00% 544 100.00% 490 100.00%

FIG. 1 represents the results of the testing of the green density ofExamples A, B and C of the subject skin coating formulation compared toa commercial ceramic paste skin coating product. A flat plate of each ofthe skin coating materials was prepared to a thickness of a fewmillimeters. The volume and the weight of the plates were measured, andtheir densities calculated. Each of the subject skin coatingformulations exhibited a higher green density than the commercialmaterial control sample. Higher density provides strength and improvedresistance to absorption of catalyst coating material.

FIG. 2 represents the results of the testing of the viscosity ofExamples A, B and C of the subject skin coating formulation compared toa commercial ceramic paste skin coating product. Viscosity was testedwith a standard Brookfield viscometer, using a number 7 spindle at 1rpm. As shown in the graph, the viscosity measurement of this materialmay have a variability of about +/−15%. Nevertheless, each of theexemplified subject skin coating formulations exhibited a lowerviscosity than the commercial material control sample. Lower relativeviscosity allows for easier pumping of the skin coating in formulationproduction and application to the substrate.

FIG. 3 represents the results of the testing of the modulus of rupture(MOR) after treatment under various conditions of Examples A, B and C ofthe subject skin coating compared to a commercial ceramic paste skincoating product.

The samples from Examples A, B and C were heat treated to simulate skincoating application conditions and to simulate process conditions(acid/base treatment and heat treatment) during catalyst coating steps.A 4 point MOR test was performed according to ASTM C880. Specifically,referring to FIG. 3, the first respective bar of each sample shows theresults of the MOR test when each sample was tested green, the secondbar of each sample shows the results of the MOR test when each samplewas tested after heat treatment, and the third bar of each sample showsthe results of the MOR test after each sample was heat treated,acid/base (alkali) washed, and fired a second time.

Each of the subject skin coating formulations exhibited a higher modulusof rupture than the commercial material control sample, when testedgreen, after heat treatment, and after acid/base (alkali) treatment anda second heat treatment.

Overall MOR strength was higher for Examples A, B and C versus thecomparative product even after heat treatment. No significant MORstrength drop was exhibited in Examples B and C formulations after heattreatment. Even when there was a drop, the percentage MOR drop was muchlower for Examples A, B and C versus the comparative product followingheat treatment

Overall MOR strength was higher for Examples A, B and C versus thecomparative product after acid and base soak followed by heat treatment.The percentage MOR strength drop was much lower for Examples A, B and Cversus the comparative product after acid and base soak followed by heattreatment.

The thermal expansion coefficient, tested between 20 and 900° C., wasmeasured for Examples B and C, at 36×10⁻⁷ and 40×10⁻⁷ respectively,compatible with commercial ceramic honeycomb substrates.

Components of the skin coating formulations may be present in thefollowing amounts by weight: refractory ceramic fiber or biosolubleinorganic fiber, from about 15 to about 50%; viscosity modifier, fromabout 0.15 to about 0.5%; colloidal inorganic oxide, from about 2 toabout 20%; inorganic particulate, from 0 to about 40%, inorganic binder(clay) from 0 to about 10%; secondary inorganic fiber, from 0 to about10% and, water from about 25 to about 50%. In certain embodiments, thecomponents may be present in the amounts by weight of: refractoryceramic fiber or biosoluble inorganic fiber, from about 20 to about 40%;viscosity modifier, from about 0.25 to about 0.4%; colloidal inorganicoxide, from about 5 to about 10.5%; inorganic particulate, from about 25to about 37%, inorganic binder (clay) from about 1.5 to about 5%;secondary inorganic fiber, from about 1.15 to about 5% and, water fromabout 29 to about 47%.

Additional skin coating material formulations were successfully preparedand are reported in Tables 2-5, set out below.

TABLE 2 Ingredient Ex. D % Ex. E % Fiber - RCF QF grade 0.00% 52.7513.98% Fiber - RCF High Index 52.75 12.12% 0.00% Cordierite 146 33.54%140 37.11% Bentonite clay 12 2.76% 12 3.18% ⅛″ glass fiber 5 1.15% 51.33% Methyl Cellulose 1.5 0.34% 1.5 0.40% Colloidal silica 38 3.49% 768.06% Water 180 46.59% 90 35.94% Mass Solids 435 100.00% 377 100.00%

A 4 point MOR test was performed according to ASTM C880 for Examples Dand E of the subject skin coating formulation as described above. Thegreen MOR for Example D was 603 psi, and the acid/heat treated MOR was606.5. The green MOR for Example E was 1147.9 psi, and the acid/heattreated MOR was 479.8.

TABLE 3 Ingredient Ex. F % Ex. G % Fiber - ISOFRAX ® (Ball milled) 20036.76% 100 20.41% Cordierite 140 25.74% 140 28.57% Calcined Kaolin 101.84% 20 4.08% E-glass - ⅛″ 0 0.00% 3.5 0.71% E-glass - 1/16″ 2.5 0.46%0 0.00% Methyl Cellulose 1.5 0.28% 1.5 0.31% Colloidal silica 80 14.71%125 25.51% Water 110 20.22% 100 20.41% Mass Solids 544 100.00% 490100.00%

TABLE 4 Ingredient Ex. H % Ex. I % Ex. J % Ex. K % Ex. L % Fiber-RCF QFGrade 80  20.59% 80  20.65% 53  14.70% 53 14.76%   0.00% Fiber-RCF HighIndex  0.00%  0.00%  0.00% 0.00% 52.75  12.12% Cordierite 140  36.04%140  36.13% 140  38.83% 140 39.00%  146  33.54% Volclay 12  3.09% 12 3.10% 12  3.33% 12 3.34% 12  2.76% HSA Fiber 5  1.29% 2  0.52% 2  0.55%0.00% 5  1.15% E-glass Fiber  0.00% 2  0.52% 2  0.55% 0.00%  0.00%Silica Fiber  0.00%  0.00%  0.00% 2.5 0.70%  0.00% Methyl Cellulose 1.5 0.39% 1.5  0.39% 1.5  0.42% 1.5 0.42% 1.5  0.34% Colloidal Silica 80 8.24% 80  8.26% 80  8.88% 80 8.91% 38  3.49% Water 70  30.37% 70 30.45% 70  32.73% 70 32.87%  180  46.59% Mass Solids 389 100.00% 388100.00% 361 100.00% 359 100.00%  435 100.00%

TABLE 5 Ingredient Ex. M % Ex. N % Ex. P % Ex. R % Ex. S %Fiber-ISOFRAX ® Ball Milled 10  3.42% 20  6.50% 30  9.30% 40  11.86% 0.00% Fiber-ISOFRAX ® High Index  0.00%  0.00%  0.00%  0.00% 52.7511.36% Silcon Carbide 146  49.91% 146  47.48% 146  45.27% 146  43.26%140 30.16% Volclay 12  4.10% 12  3.90% 12  3.72% 12  3.56% 12  2.58%E-Glass ⅛″ fiber 5  1.71% 5  1.63% 5  1.55% 5  1.48% 5  1.08% MethylCellulose 1.5  0.51% 1.5  0.49% 1.5  0.47% 1.5  0.44% 1.5  0.32%Colloidal Silica-1034a 38  5.20% 38  4.94% 38  4.71% 38  4.50% 38  3.27%Water 80  35.15% 85  35.06% 90  34.98% 95  34.90% 215 51.22% Total Mass293 100.00% 308 100.00% 323 100.00% 338 100.00% 470 100.00

It will be understood that the embodiments described herein are merelyexemplary, and that one skilled in the art may make variations andmodifications without departing from the spirit and scope of theinvention. All such variations and modifications are intended to beincluded within the scope of the invention as described hereinabove.Further, all embodiments disclosed are not necessarily in thealternative, as various embodiments of the invention may be combined toprovide the desired result.

1. A skin coating material for porous ceramic substrates comprising:refractory ceramic fiber or biosoluble inorganic fiber; a viscositymodifier; a colloidal inorganic oxide; optionally, an inorganic binder;optionally, an inorganic particulate; and, optionally, a secondaryinorganic fiber.
 2. The skin coating material of claim 1 wherein therefractory ceramic fibers or biosoluble inorganic fibers comprise atleast one of aluminosilicate fibers, alkaline earth silicate fibers, orcalcium aluminate fibers.
 3. The ceramic paste composition of claim 2wherein the alkaline earth silicate comprises at least one of magnesiumsilicate or calcium magnesium silicate.
 4. The skin coating material ofclaim 1 wherein the inorganic particulate comprises at least one ofalumina, cordierite, mullite, titania, aluminum titanate, or siliconcarbide.
 5. The skin coating of claim 1 wherein the inorganic bindercomprises an uncalcined clay or a calcined clay.
 6. The skin coatingmaterial of claim 5 wherein the clay comprises at least one ofattapulgite, ball clay, bentonite, hectorite, kaolinite, kyanite,montmorillonite, palygorskite, saponite, sepiolite, sillimanite, orcombinations thereof.
 7. The skin coating material of claim 1 whereinthe viscosity modifier comprises at least one of alkyl cellulosepolymers, polyalkylene oxides, polysaccharides, polyacrylic acids,polyacrylamides, and mixtures thereof.
 8. The skin coating material ofclaim 1 wherein the alkyl cellulose polymers comprises at least one of,methyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methylcellulose, hydroxyethyl cellulose, carboxy methyl cellulose,hydroxyethyl carboxymethylcellulose, orcarboxymethylhydroxyethylcellulose, or mixtures thereof.
 9. The skincoating material of claim 1 wherein the colloidal inorganic oxidecomprises at least one of colloidal silica, colloidal alumina, colloidalzirconia or mixtures thereof.
 10. The skin coating material of claim 1wherein the secondary inorganic fibers comprise at least one of glassfibers, leached silica fibers, high alumina fibers, mullite fibers,magnesium aluminosilicate fibers, S-2 fibers, E-glass fibers, basaltfibers or fine diameter alumina-silicate fibers.
 11. A skin coatingmaterial for porous ceramic substrates comprising: refractory ceramicfiber or biosoluble inorganic fiber; a viscosity modifier; a colloidalinorganic oxide; an inorganic binder; an inorganic particulate; and, asecondary inorganic fiber.
 12. The skin coating material of claim 11comprising refractory ceramic fiber.
 13. The skin coating material ofclaim 11 comprising biosoluble magnesia silicate fiber.
 14. The skincoating material of claim 11 comprising a methyl cellulose viscositymodifier, colloidal silica, an inorganic binder, and cordieriteparticulate.
 15. The skin coating material of claim 14 wherein theinorganic binder comprises at least one of calcined kaolin, bentoniteclay or volclay.
 16. The skin coating material of claim 11 wherein thesecondary inorganic fiber comprises E-glass fiber.
 17. The skin coatingmaterial of claim 11, further comprising at least one of an organicbinder fiber, an organic binder, or a resin.
 18. The skin coatingmaterial of claim 11 comprising a methyl cellulose viscosity modifier,colloidal silica, an inorganic binder, and silicon carbide particulate.19. The skin coating material of claim 18 wherein the inorganic bindercomprises volclay.
 20. The skin coating material of claim 1, furthercomprising at least one of an organic binder fiber, an organic binder,or a resin.
 21. A method of producing a porous ceramic substrate skincoating, comprising forming a mixture of: ceramic fibers or biosolubleinorganic fibers; a viscosity modifier; a colloidal inorganic oxide;optionally, an inorganic binder; optionally, an inorganic particulate;and optionally a secondary inorganic fiber.
 22. The method of claim 21,wherein said forming a mixture comprises: forming a dry mixture of:ceramic fibers or biosoluble inorganic fibers; a viscosity modifier;optionally, an inorganic binder; optionally, an inorganic particulate;and optionally a secondary inorganic fiber; forming a wet mixture of acolloidal inorganic oxide and water; and mixing the dry mixture and thewet mixture.